1. Field of the Invention
The present invention relates to a circuit switched wireless communication system such as an anti-disaster wireless communication system, and more particularly to a wireless packet transfer apparatus for transferring a packet by wireless by using Internet Protocol (IP).
2. Description of the Related Art
With the recent advancement of IP networks, various applications are provided by using IP. It is expected that seamless communications of IP are realized even among different types of networks such as wired and wireless networks, by adopting new technologies such as mobile IPv6 (IP version 6).
A public utility wireless communication system such as an anti-disaster wireless communication system generally adopts a circuit switching system. Using IP for wireless communication systems of this type is associated with the following problems. The first problem is that it takes a long time for data transmission because a transmission bandwidth is narrow (about 25.6 Kbit/s at a maximum transmission speed). The second problem is a throughput lowered by re-transmission caused by packet loss and a reduction in a window size of Transmission Control Protocol (TCP) and the like, because of a high bit error rate (10−3 to 10−4) of a transmission line. The third problem is a fixed error correction encoding rate of each call because of circuit switching. Since a predetermined error correction encoding is performed for data transmission even in a good quality state of a wireless propagation path, the transmission bandwidth cannot be used efficiently. If moving image transmission is performed by using, for example, Moving Picture Experts Group 4 (MPEG4), this transmission is processed like file transfer which does not permit any error, although MPEG4 itself has an error proof function of about 10−3 to 10−4. The communication service quality such as a moving image display rate is therefore degraded. The fourth problem is that if a router or the like is used as an interface between wireless apparatus and a network, an error packet is unconditionally discarded because of a data link protocol of the router. Therefore, even if an application having a bit error permission function, such as MPEG4, is used, the whole packet is lost because of a bit error so that the image quality is degraded.
As technologies capable of solving the first problem, header compression technologies are known. The header compression technologies are described in the documents Request for Comment (RFC) compiling standardized specifications of Internet Engineering Task Force (IETF). TCP/IP header compression technologies are written in RFC1144, and IP/UDP/RTP header compression technologies are written in RFC2508. Although these header compression technologies are effective for Voice over IP (VoIP) packets having a short packet length, they provide less compression effects for long packets used in most of communications. As the technologies for solving the second problem, wireless TCP has been proposed. In a gateway apparatus disposed between a wired section and a wireless section, the wireless TCP performs division of a TCP connection and proxy re-transmission of TCP by mounting an agent. However, the wireless TCP cannot provide a TCP connection of end-to-end and the mount of an agent is complicated.
In mobile communications, the technologies have been proposed in which wireless parameters for error correction, modulation and the like are adaptively set in accordance with Quality of Service (QoS) (e.g., refer to JP-A-2003-259447).
The technologies described in JP-A-2003-259447 are directed to wide band mobile communications over 2 Mbps or faster ultra high speed mobile packet transmission systems, and do not consider a circuit switched wireless packet transmission system having a relatively low transmission speed to which the present invention is applied.